In order to secure a workpiece on a table so that it can be machined or otherwise worked on, it is standard to form the table with an array of slots formed as inverted T's as seen in section. The workpiece is laid on the surface and one or more stops are fitted to the slot on one side of the workpiece and one or more clamps to the slot on the other side of the workpiece.
Each stop typically comprises a so-called T-nut which fits in and can be slid along the respective T-slot. A stop block sitting atop the table is secured by a bolt passing through it and extending into a threaded bore of the respective T-nut. Thus the stops can be slid along the table and bolted in any position along the slot.
Each clamp normally has a T-nut like the stops and a clamp body formed with a hole through which the shank of a clamp bolt passes with some play. The bolt has an eccentric head that fits in a complementary seat on the clamp block so that as the screw is rotated the clamp block will move radially of the bolt axis. Thus the clamp bolt can be rotated to brace the clamp block tightly against its side of the workpiece.
To provide some degree of adjustability, the stop blocks can be formed with slots instead of a simple cylindrical hole, allowing them to be bolted down in different positions. This system has the disadvantage that the stop can, in conditions of extreme stress, shift and thereby let the workpiece move. In another system so-called step blocks are used. These items are two-part stop blocks with stepped interfitting surfaces so that they can be shifted relative to each other to, in effect, make a stop block of different widths. Such an arrangement is fairly clumsy and has extra parts that are easily lost.